Compact fluorescent lamps have been developed as replacements for incandescent lamps because fluorescent lamps are considerably more efficient and have longer operating lives than incandescent lamps. A compact fluorescent lamp has roughly the shape of an incandescent lamp and can be installed in a conventional lamp socket. Prior art compact fluorescent lamps have had one or more disadvantages. A lamp ballast circuit is required, since fluorescent lamps cannot be operated directly from line voltage. The ballast circuit adds to the cost of the lamp and must be packaged in the base of the lamp. Furthermore, the form factor of the fluorescent lamp tube is less than ideal for use as an incandescent lamp replacement because the light emitted per unit volume is maximized in a long, small diameter fluorescent lamp tube.
Current compact fluorescent lamps use a twin tube or double twin tube architecture. Twin tube fluorescent lamps typically include a pair of straight tubes that are interconnected at or near one end to form a generally U-shaped tube. Conventional twin tube fluorescent lamps have filament cathodes at opposite ends, and a continuous discharge extends between the cathode filaments. A compact fluorescent lamp using a frequency converting ballast to convert 60 Hz line voltage to a frequency in the range of 20-30 kHz is disclosed in U.S. Pat. No. 4,857,806 issued Aug. 15, 1989 to Nilssen. The high frequency output of the ballast is applied to the lamp filaments.
Compact fluorescent lamps utilizing electrodeless fluorescent lamps have been disclosed in the prior art. A closed loop magnetic core transformer, contained in a reentrant cavity in the lamp envelope, induces a discharge in an electrodeless fluorescent lamp in U.S. Pat. No. 4,005,330 issued Jan. 25, 1977 to Glascock et al. Discharge is induced by a magnetic core coil within the envelope of an electrodeless fluorescent lamp in the light source disclosed in U.S. Pat. No. 4,017,764 issued Apr. 12, 1977 to Anderson. In both of the above-mentioned patents, the operating frequency is limited to about 50 kHz because of the lossy nature of magnetic materials at high frequency. An electrodeless fluorescent light source utilizing an air core coil for inductive coupling at a frequency of about 4 MHz is disclosed in U.S. Pat. No. 4,010,400 issued Mar. 1, 1977 to Hollister. However, such a light source has a tendency to radiate at the power of operation and exhibits nonuniform plasma excitation.
An electrodeless fluorescent light source, utilizing frequencies in the 100 MHz to 300 GHz range, is disclosed by Haugsjaa et al in U.S. Pat. No. 4,189,661 issued Feb. 19, 1980. High frequency power, typically at 915 MHz, is coupled to an ultraviolet producing low pressure discharge in a phosphor coated electrodeless lamp which acts as a termination within a termination fixture.
A compact fluorescent light source wherein high frequency power is capacitively coupled to a low pressure discharge is disclosed in U.S. Pat. No. 4,266,167 issued May 5, 1981 to Proud et al. The lamp envelope has an outer shape similar to that of an incandescent lamp. An outer conductor, typically a conductive mesh, is disposed on the outer surface of the lamp envelope, and an inner conductor is disposed in a reentrant cavity in the lamp envelope. Frequencies in the range of 10 MHz to 10 GHz are suggested. An electrodeless discharge tube wherein high frequency energy is coupled to a discharge through external electrodes is disclosed in U.S. Pat. No. 4,798,997 issued Jan. 17, 1989 to Egami et al. Another electrodeless fluorescent light source which is energized by a high frequency power source is disclosed in U.S. Pat. No. 4,427,923 issued Jan. 24, 1984 to Proud et al. In all of the lamps that operate at high frequency, it is important to minimize radiation of the high frequency energy.
It is a general object of the present invention to provide improved fluorescent light sources.
It is another object of the present invention to provide a compact fluorescent lamp that is suitable for replacement of an incandescent lamp.
It is a further object of the present invention to provide a fluorescent light source wherein a twin tube fluorescent lamp is capacitively driven by RF electrical energy.
It is yet another object of the present invention to provide a RF driven compact fluorescent lamp which produces very little RF radiation.
It is still another object of the present invention to provide fluorescent light sources which are low in cost and easy to manufacture.